A generally-known film covered battery has a battery element and an exterior film for hermetically sealing the battery element. The exterior film has at least a heat-fusion bonding layer, a metal layer, and a protective layer.
In a manufacturing process of the film covered battery having the previously-discussed structure, a very small hollow may occur on the surface of the film covered battery due to an external force. For instance, when the film covered battery is picked up by means of a robot hand, a very small hollow may occur on the surface of the protective layer of the exterior film.
In order to prevent the surface of a laminated film from being damaged or stained during the manufacturing process, Patent document 1 teaches a method of manufacturing a laminated battery using a laminated film with a protective film attached to a protective layer. In the Patent document 1, the protective film, whose adhesive strength decreases by ultraviolet irradiation, is attached to the surface of the laminated film in advance, and a film covered battery is manufactured, using a battery element as well as the laminated film, and thereafter only the protective film is removed by ultraviolet irradiation.
However, the previously-discussed method requires a process for exfoliating the protective film. Assuming that a hollow has already occurred on the protective layer before attaching the protective film, such a hollow is exposed by exfoliating the protective film. In contrast, assuming that, for the purpose of covering the hollow, the protective film is left on the surface of the laminated film, in the Patent document 1 the thickness of the film covered battery increases because of the protective film attached over the entire surface of the protective layer of the laminated film.
In many cases, a film covered battery is used in a state where a large number of film covered batteries are stacked up in their thickness directions. Thus, even when there is a slight increase in thickness of a single film covered battery, the total thickness of the stacked or assembled film covered batteries greatly increases.
It is an object of the invention to cover a hollow occurring on a surface of a film covered battery without increasing the thickness of the film covered battery.